From the earliest forms of transportation on water, various types of propulsion systems have been employed on watercraft. Boats have been propelled by oars, paddle wheels, propellers and, in modem times, jet drive systems.
The propeller replaced both the side and stem paddle wheels about 150 years ago. It is still a primary method of providing thrust, even on huge modem aircraft carriers and cruise ships. But the fast-moving, sharp blades, usually made of polished stainless steel, can be most unsuitable for the propulsion of small recreational boats at beaches, lakes or rivers where swimmers, children and novice watercraft operators intermix. The safety hazards of propeller-driven watercraft in such contexts has contributed to the popularity of modem jet drive systems for small watercraft, including Personal Water Craft (PWC), such as jet skis, WaveRunners™ (a trademark of Yamaha Corporation) or the like.
Watercraft that are powered by jet drive systems are typically steered by a moveable sleeve placed around the jet stream, pivotal on a vertical axis, with a linkage connection to the handlebars or a steering wheel. Turning this sleeve deflects the jet stream and steers the boat. However, if the power is turned off, there is no jet stream to effect steering, even though the watercraft may still be moving at a considerable speed while slowing down. In such instances, momentum can cause the watercraft to simply continue straight on its path at the time the jet stream was cut off, while decelerating. The lack of steering control in such instances, can result in safety hazards and can take new operators by surprise.
Watercraft drive systems often include one or more propellers or jet pumps with impellers that are driven by an engine. Propellers and jet impellers can become less efficient at high speeds, such as the speeds at which many modern engines run at peak power output. Common outboard and stern drive systems typically include relatively expensive bevel gears between the engine and the propeller, which are used to reduce the engine speed at the propeller. However, jet drive systems typically include a jet pump that is directly coupled to the engine, where it may not be economical to interpose reduction gears between the pump and the engine.
Another problem for watercraft manufacturers is that the molds for the hull of small boats are relatively expensive. To help minimize costs, manufacturers often employ a basic hull that is common to several layouts of deck, interior plans and power choices, both size and type of outboard, stem drive or jet.
The shape of the hull (the “V” shape) contributes to the directional stability of the boat. Typically, the sharper the hull shape, the more stability, but at the expense of lower top speed and higher fuel consumption. To improve speed, fuel efficiency or both, the “V” shape of some hulls have been made as flat as possible and skegs or fins have been employed to improve stability. However, there should be nothing protruding below the water line in a jet-powered boat, so the same hull design which is stable at top speed with an outboard or stem drive, may not be as directionally stable when fitted with a jet drive system.